This invention relates generally to laser modulator drivers, and more specifically to biasing the outputs of a laser modulator driver with a nearly constant output impedance.
High speed optical transmission networks often rely on pulses of light in order to transmit data. Lasers are often used to provide a light source. Rather than modulating the laser itself, a separate modulator is sometimes used. The laser is operated in continuous wave mode and the laser light is then passed into a modulator. The modulator varies the amount of light provided to an output in order to generate the pulses of light.
Modulator drivers generally provide a large voltage swing to a laser modulator in order to turn the modulator on and off completely. However, laser modulators often introduce undesirable frequency modulation, or chirp, into the output signal. In order to reduce the amount of chirp, a bias voltage is usually applied to the laser modulator. The bias voltage is often applied on top of the modulation voltage. That is, the bias voltage serves as a DC offset for the modulation voltage.
In many cases the bias voltage is applied to an output of the modulator driver. However, this may introduce additional components in a high speed path. The additional components in the high speed path often limit the maximum speed at which the modulator can operate. Additionally, bias components coupled to the output of a modulator driver often introduce parasitics, such as capacitance, to the output of the modulator driver. These parasitics often degrade the output parameters of the modulator.